Gel package structural enhancement of compression system board connections

ABSTRACT

A MCM system board uses a stiffener arrangement to enhance mechanical, thermo and electrical properties by incorporating an LGA compression connector in a computer system. The present designs of large scale computing systems (LSCS) in IBM use a MCM that is attached to a system board and held together by a stiffening frame. Due to the nature of the manufacturing of the system board, there can be significant gaps formed in the mounting area of the MCM between the board and the stiffener. A method is described that not only fills the void, it also, in addition promotes thermo conduction of excess heat away from the MCM and at the same time promotes enhanced electrical properties of the LGA connections of the MCM to the system board.

TRADEMARKS

IBM® is a registered trademark of International Business MachinesCorporation, Armonk, N.Y., U.S.A. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to a gel package structural enhancement ofcompression system board connections, and particularly to themechanical, electrical, and thermo improvements to computer systems thatutilize LGA compression connector techniques on system boards.

2. Description of Related Art

In the present construction of the IBM Z-series systems as well as inother programs such as the IBM P-series, the Multi-chip module (MCM) iscompressed to the main system board using a mechanical encapsulated MCMstructure and a Land Grid Array (LGA) type connection. There hasdeveloped with the advent of large scale computing systems, like thesementioned, that a unique problem can be recognized in the constructionof the PCB system board. When the system board is fabricated, a changein thickness is produced in the areas of large arrays of vias or platedthru holes (PTH). With this change in thickness, support for acompression style connection scheme such as a Land Grid Array (LGA) cancause many problems. For one, a significant space or gap will developbetween the system board and the supporting fixture or stiffener. Thisgap would then cause instability or flexing of the board, due toexpansion and contraction produced by the thermo properties of thesystem board. To compound this problem, the arrangement of the highdensity of PTH in the system board due to the design of the I/O patternof the MCM would cause an uneven pattern on the system board. In areasof low via density, the board would not contract in size as it would inareas of high via densities. In the present design of IBM's Z and Psystem series, the array of I/O's on the MCM is divided into four areas.This is done for several reasons, one of which is to permit thefabrication of the LGA connector used between the MCM and the systemboard. This then causes the system board to have different thicknessesthat match the I/O pattern.

The resulting board uses then a unique insulator placed between theboard and the holding fixture. At present, laminations of multiplelayers of insulator arranged in a stacked pattern are used to compensatefor the board's thickness change. This approach does not provide a fullysupporting structure and also is limited to what size gap can be filled.If the gap exceeds a predetermined size, then the board would not beacceptable. It would be impossible to have multiple stacked insulatordesigns for use in the assembly of the computer system. At present thesystem boards are then subjected to screening so that the gap is of aspecific size, so that the presently designed stacked insulator isusable.

Furthermore, to achieve the best mechanical interface and electricalconnections, it is required that the system board along with itsstiffener be as flat as possible in the LGA area. This requirement cansignificantly increases the total cost of the delivered boards from thePCB vendor as well as the overall cost to produce the computer system.The stringent requirement for board flatness will result in the sortingof the PBC boards based on measurements made after manufacturing. Inaddition, to this, to insure a sound mechanical interface, all thesupporting mechanical structures such as the MCM mounting hardware, andstiffener have to be controlled to very tight tolerances. At presentthere is a significant gap between the bottom side of the system boardand the stiffener. This is caused by a change in thickness of the systemboard in the area of the MCM interface.

The main reason for the thickness difference of the printed circuitboard (PCB) in the LGA area is due to the number of plated thru holes(PTH) or vias in that area. With a large number of vias in any givenarea shrinkage occurs on both sides of the PCB board. The larger thenumber of vias in any area on the board, a corresponding largershrinkage area is produced. For some MCM designs, the I/O arrays arearranged in separate array groups on the bottom side of the MCM. On thesystem board, there would be a corresponding arrangement of plated thruvias to permit electrical connections within the system board, and thusthe system board will be thicker in the areas where there are fewer orno vias and thinner in areas with a large number of vias. Thus dependingon the design for the I/O locations for the MCM it would be possible fora system board to contain a non-uniform thickness or rippled effect inthe LGA area. At present, the non-planer area directly under the MCM onthe topside of the system board is compensated in part, by the pliableLGA connector structure, but the area under the MCM on the stiffenerside is not compensated for at all. Although there is an insulatorbetween the system board and the stiffener, the void created by thethickness change in the LGA area is not compensated for. From mechanicalanalysis, it has been found that the system board will tend to have aconcaved bend or thickness change of up to 0.008 inches and in someinstances a large gap of 0.012 inches are created. In the LGA area, theboard could also have a profile that is rippled in shape. The shape andsize of the bend is variable and not always uniformed, and thus the voidcannot be filled without some structural design changes. Studiesindicate that the system board tends to form a dish or concave profilein the area of the high density via area.

There have been a number of suggestions to date that include a rigidpreformed crowned pad that would be inserted in addition between theinsulator and the system board or for additional sheets of Mylar in astack in the crowned area. Using this preformed crowned pad or any othersuch design to compensate for the irregularities in the LGA area wouldwork for a dish or concave shape of one size, but not for an uneven orstructures with multiple topology differences. With the natural creationof the void or dishing on the system board, great care must be used inthe overall design to insure component and mechanical integrity. To helpprevent the MCM module from breaking, all the mechanical uncertaintiesand tolerances have to be controlled in the assembling of the functionalpackage. To insure that the LGA interface structure is in the bestelectrical contact, i.e. low contact resistance, the support areabetween the system board and the stiffener must be as rigid and solid aspossible. If the mechanical system, for mounting the MCM and theelectrical interface, is incorrectly compensated for, the gap on thesystem board could cause significant module damage or electrical contactproblems. This would include cracking of the MCM or poor electricalconnection through the LGA interface. At present, all suggestions fallshort in the solution of these problems.

SUMMARY OF THE INVENTION

In accordance with the invention, we have developed a gel package ofdoughy or gel or gel paste like material formed by a first layer of filmand a second layer of film enclosing our non-compressible gel pasteformulated to become stiff after application of pressure causes saidfirst and second layer to compress our non-compressible gel paste. Thisgel package, alone or with its framework can be manufactured and usedwith our MCM final assembly where application of pressure is acompression caused by mounting an MCM module to a PCB system boardagainst a mechanical stiffener when our gel package is placed betweenthe PCB system board and the mechanical stiffener and the MCM module andmechanical stiffener are drawn together compressing the gel packageagainst the PCB system board.

The gel package allows for compensation of irregular thickness changesdue to the design of the system board. The gel package allows use of ournew methods of use of the gel package we developed and will describe. Wefill the gap between the system board and its supporting hardware with apackage of non-compressible gel or suitable insulating substance. As wewill also illustrate, we have provided utilization of a non-compressiblefluid like material in a package used as a replacement for the insulatorsheets used between the system board and its stiffener. This insulatorpackage could be composed of a lamination of Mylar® polyester film,Kapton® polyimide film (registered trademarks of E. I. DU PONT DENEMOURS AND COMPANY), or some other suitable material encapsulatingnon-compressible materials including but not limited to thermo paste asused between the IC chips on the MCM such as Dow Corning TC-5022 or athermo conductive, low temperature curing epoxy. In addition, thenon-compressible material could be composed of any non-toxic insulatinggel that would have the desired characteristics that would insure fullcontact support between the system board and the mechanical stiffener.This insulating package of material would conform to all theirregularities that are present between the system board and themechanical stiffener used to support the board and MCM components. Inaddition, if a thermo paste were used, heat removal from the bottom ofthe system board in the area of the MCM would be possible. If the gel orresin material had a low temperature-curing characteristic that hardenswhen it is heated by the temperature rise of the board in the powered onsystem, it would form a solid structure between the board and thestiffener. This harden insulating package would require, at a latertime, a new cushion to be used if the board were to be removed from thestiffener. The new arrangement enhances the overall structure of theMCM-LGA-system board-stiffener design. At the same time there isprovided an enhancement to the electrical properties of the MCMinterconnects, and an enhancement for the removal of heat generatedunder the MCM. This also reduces the need for costly sorting the PCBboards and at the same time, reduces the added expense for the costlymachining of the MCM structures. Thereby the tolerances needed forassembly can be relaxed and the total cost for the completed systemreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 a illustrates one example of the irregularities formed betweenthe system board and the mechanical stiffener with standard stackedinsulators under board.

FIG. 1 b illustrates one example of the irregularities formed betweenthe system board and the mechanical stiffener filled withnon-compressible gel package.

FIG. 1 c illustrates in detail the irregularities formed between thesystem board and the mechanical stiffener with standard stackedinsulator under board.

FIG. 2 illustrates one preferred embodiment of the insulating gelpackage

FIG. 3 illustrates one example of an exploded view of the completeassembly

FIG. 4 a. illustrates one example of the gel package using a doubleframe for holding the gel package

FIG. 4 b. illustrates one example of the gel package using a doubleframe for holding the gel package as used in the system

FIG. 5 a. illustrates one example of the gel package using a singleframe for holding the gel package

FIG. 5 b. illustrates one example of the gel package using a singleframe for holding the gel package as used in the system with microchannel

FIG. 5 c illustrates a detail of the expansion channel.

FIG. 6. illustrates one example of the gel package using a double framefor holding the gel package as used in the system with micro channel.

FIG. 7. illustrates one example of the gel package using a double framefor holding the gel package as used in the system with micro channel andscrew for pressure/volume adjustment.

FIG. 8. illustrates one example of the gel package with an imbeddedpackage of epoxy curing material and method for activation by amechanical screw.

FIG. 9 illustrates one example of the gel package with an embeddedpackage of curing material and method for activation by pressure fromassembly forces.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1 a, the current assembly technique requires aninsulator (80) and pad stack (81) under the LGA (30) contact patternfield that is between the system board (20) and the stiffener (10). ThePCB board (20) in this example has a concave or dish shape (21) in theLGA mounting area. This unevenness or dish shape (21) is created duringthe manufacture process of the board and it's shape and size ispartially due to the board's power/signal cross section. The change inthickness is also dependent on the number of vias (25) and crosssupports designed into the board. As can be seen in the detail FIG. 1 cthere is a stair case effect as the insulators are stack upon eachother. As the density of the vias (25) increase, so will change theheight of the gap. The solution to the previously described problemwould be to replace the insulator (80) and pad stack (81) with anon-compressible gel package (60) as shown in FIG. 1 b. This would theninsulate the system board (20) from the stiffener (10) as well as toprovide a mechanical supporting structure that conforms to allirregularities between the system board (20) and the stiffener (10).

With the proposed solution, the gap (21) between the stiffener (10) andthe system board (20) is filled with a self-conforming insulatorstructure (60) shown in FIG. 2 that is shaped like a cushion. Thiscushion (60) comprises of two separate layers (64,65) of insulatingmaterial. In FIG. 2 this insulating material is a material such as Mylarpolyester or Kapton polyimide envelope is filled with non-compressiblegel paste material such as a silicon gel, thermo-conducting paste, orepoxy resin like substance (63) that conforms to all the irregularitiesand applies equal contact pressure on the board surface after assembly.The gel or resin material (63) can in a preferred embodiment becomprised of a low temperature-curing epoxy so that it would harden whenit is heated by the temperature rise of the board in the powered onsystem. At a later time, a new cushion (60) would be used if the board(20) were to be removed from the stiffener (10) . The advantage of thisembodiment, in it's several alternative embodiments is to enhance theoverall structure of the MCM-LGA-system board-stiffener design such thatthere was an enhancement to the electrical properties of theinterconnects, and an enhancement for the removal of heat generatedunder the MCM. The application will also reduce the costly sorting thePBC boards and at the same time, the costly machining of the MCMstructures of the tolerances needed for assembly could be relaxed. Thusthe total cost for the completed system would be reduced.

As will be seen in various embodiments of the non-compressible gel pasteformulated for the purpose from non-compressible gel paste material suchas a silicon gel, thermo-conducting paste, or epoxy resin like substance(63) that conforms to all the irregularities and applies equal contactpressure on the board surface after assembly, the gel paste (could be agel or pasty viscosity) compensates for the irregular thickness changesdue to the design of the system board. With the gel package we fill theresulting gap between the system board and its supporting mechanicalstiffener with a package of non-compressible gel or suitable insulatingsubstance. This insulator package consists of a lamination of two sheetsof films to form the envelope, such as films of Kapcon or Mylarmaterials that encompasses and envelopes a non-compressible insulatingsubstance such as the thermo paste. In the preferred embodiment this isheld within a supporting frame. The resulting package is used betweenthe system board and the supporting mechanical hardware presently knownas a stiffener or mechanical stiffener. The advantage of this solutionis the fact that all the gaps created by the design of the system boardare filled independent of a fixed separation dimension or if there aremultiple gaps of different sizes. As the preferred embodimentillustrated in FIG. 2 shows, in order to compensate for the irregularthickness changes due to the design of the system board, we provide thepackage (60) which still comprises two laminates (64, 65) that aresupported and held together using a frame (66). Between the twoinsulating layers there is a deposit of a non-compressible material (63)such as thermo paste or an uncured epoxy. The design of this FIG. 2package (60) can use the same design as that of the LGA connector systemshown in FIG. 1 b and FIG. 3 to help facilitate assembly with thisembodiment's application. The frame (66) (one or two elements of theframework) can have an squeeze zone within the weld area to provide asmall relief area for the gel paste when compression occurs. Thisalternative is shown by the line in FIG. 2 around the gel paste area offrame (66). The application is in support of but not limited to thepresently described alternative embodiment. It is anticipated that also,as an example, it could be used as an insulating shock mount betweendissimilar structures to compensate for mechanical movement and designmismatching.

FIG. 3 shows the exploded view of the complete structure of board (20),stiffener (10), MCM (40) and one example of the compliant gel packagestructure (60).

Description:

For the design of the load-compensating gel package (60), attention mustbe given on how the excess volume of non-compressible material (63) ishandled as well as providing structural integrity of the gel package(60). There are several methods that can be used. The first is asfollows. Referring to FIG. 4 a, the construction of the package is suchthat the outer frame of the package is designed to be slightly less inheight as the crown of the gel compensator. The frame design (50) issuch that the thickness of the frame is equal on both sides of the gelpackage (60). When the gel compensator (60) is installed between thestiffener (10) and the system board (20) and is compressed by thecombination of the MCM (40) and the LGA (30), the excess gel will expandto the stop plates (52) on the outer frame (50). The over all design ofthis frame is such that a stand off of 0.008 inches is pre-determined.Thus, the gel package with a total height of 0.010 inches at its centeris used; the elliptical shape of the package (61) seen in FIG. 2 wouldthen re-form into the shape shown in FIG. 4 b. The stiffener (10) wouldbe machined such that there was a small step down (12) in the LGA areato house the gel package (60) and so that support would be provided tothe system board (20) wherever it was needed.

Another design would use a gel package (60) that is constructed in amanner with the frame (51) on one side of the gel package (60) forsupport as shown in FIG. 5 a. The advantage of this design would be foroverall manufacturing considerations. The stiffener (10) would then bemilled such that a channel (11) is formed in the area between the LGAconnector area and the outer most clamping area of the frame as shown inFIG. 5 b. The step (15) created with the milling of the small channel(11) would act as part of the gel package's (60) frame. As shown in FIG.5 c the gel package (60) is compressed during the mounting of the MCM(40), the excess gel material (62) would self level and fill the void(14) of the channel (11). The final results would also provide a solidbacking to the system board (20). One advantage for the use of thechannel would be to reduce the height needed between the stiffener (10)and the system board (20) and to permit a larger volume of gel (62) inthe package (60).

A third anticipated design shown in FIG. 6 would be to use the doubleframe compensator (50) as described in FIG. 4 a, b and the use of achannel (11) cut into the stiffener (10). The stiffener (10) is machinedin such a manner that the system board (20) is supported outside of theLGA area, and a channel (11) is cut to completely frame the LGA supportarea. The frame (50) on the gel package (60) would then reside withinthis channel (11) and its height would be the same as the height of thestiffener (10) to the system board (20). When the MCM (40) is compressedto the front side of the system board (23), the gel would compensate forthe uneven surface of the bottom side of the system board (22). Theexcess gel (62) would then fill the cavity (14) between the gel packageframe (50) and the cut channel (11). Again a larger volume of materialcould be used in the design to cover a larger gap and thus permit asignificant easement of the mechanical tolerances. An added embodimentto control the pressure and volume of the gel package is shown in FIG.7. An adjustment screw (70) is incorporated in the stiffener (10) at oneor at various locations around the gel package placement area. Thiswould permit micro adjustments to the fluid system to insure that thegel or other such material is optimized for supporting the system boardto stiffener interface. This technique would insure that a positivepressure of the gel package would be maintained.

To accommodate, if needed, the removal of excess heat generated underthe MCM, the gel package could contain a thermo-conducting paste similarto the paste, which is used to interface the chips to the heat sink.When used with any of the previously defined applications, the pastewould help to maintain a thermo-path to remove the excess heat to thestiffener. The reliability of the system of the system would be enhancedbecause the operational temperature of the MCM in the connector areawould be in better control and therefore system speeds and reliabilitycould be enhanced.

In certain applications, there might be an advantage to cause the gelpackage to form a permanent seal to the system board. This could beaccomplished by using a low temperature curing epoxy as part of the gelpackage design. This could actually be a thermo-conducting epoxy. Whenthe system is powered on, the temperature of the bottom side of theboard will increase due to current through the vias and the heat sinkingeffects of the contacts between the MCM and the system board. This heatincrease will then cause the resin to cure and harden in the conformedshape of the gap between the stiffener and the board providing for bothan enhanced mechanical and thermo structure. In addition, the amount ofthe epoxy hardness could be regulated such that the epoxy could be ashard as possible or very elastic in nature. The higher the elasticity,the more rubber like the bond and thus the more flexible the adhesion.This would permit different applications of the epoxy when used in theproposed solution. This is especially important when the surfaces flexfrom the effects of temperature, or stress.

One concept would be to use an adjustment screw (71) as shown in FIG. 8to activate a small package (72) of curing agent within the larger gelpackage. This could be accomplished by the use of a small sharp pointattached to the inside of the outer layer (65) of the gel package (60).The curing agent package (72) would be placed as a decal over the sharppoint during the assembly of the overall package (60). The decal couldalso contain the sharp activation point to enhance the assembly of theepoxy package (60). When the screw (71) is adjusted inward, the sharppoint would then puncture the small package (72) of curing agent andthereby releasing it into the epoxy filled package (60). This would thencause the epoxy to harden and form a solid interface between the systemboard (20) and the stiffener (10). The epoxy could also have thermoconductive properties similar to that of a thermo conductive paste. Thiswould permit the removal of excess heat from under the MCM during systemoperation and at the same time form a ridged support between the systemboard (20) and the stiffener (10).

Another concept, as shown in FIG. 9, would be to include within the gelpackage (60) an activation agent package (74) that would contain anepoxy-curing agent (75). This package (74) could be designed to rupturewhen the compression forces exceed a predetermined value during theassembly of the MCM to the system board (20) and stiffener (10). Theagent (75) would then be injected into the epoxy package (60) andthereby causing the epoxy to harden into a solid form that would conformto the system board (20) irregularities.

A third concept would be to use a pressure sensitive curing epoxy thatwould self activate when subjected to pressure such as that created whenthe MCM (40) is compressed to the system board (20).

In addition to the mechanical activation of the curing agent asdescribed in FIG. 8 and FIG. 9, the enclosed active curing agent (75)could be dispensed into the gel package (60) by physical manipulation ofthe gel package (60) by the assembler. That is to say, the operatorwould squeeze the gel package (60) by hand to cause the enclosed curingagent package (74) to rupture. This would permit the operator to intermix the epoxy and curing agent together just before assembly, therebyinsuring that the curing agent was well mixed with the epoxy.

The electrical interface between the MCM and the LGA connector issignificantly enhanced due to the fact that the system board isuniformly supported in the LGA area. Because no movement is possible ofthe system board due heat flux of the PCB material, the LGA connectorsare in the best possible contact with the MCM. With better contact ofthe LGA connectors being made, the contact resistance of the inner mostLGA connections of the array will be lowered. With a lower signalamplitude loss for the signal nets and a lower drop in voltage andbetter power distribution for the power connections, the systemperformance can be improved. At the transmission rates for current andfuture machines, every impact due to power loss or signal integrity hasan effect on the delivered system performance and thus the improvementsmade by this application will enhance system performance.

The retro fitting of the computer systems using an LGA connectioninterface could be accomplished through the use of any one of thedescribed methods with little or no modifications. Ideally, theinsulator that is presently being used on all LGA system designs wouldbe replaced by a self-compensating gel package.

While the preferred embodiments to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A method of enhancing the mechanical structure of a printed circuitboard (PCB) system board for a multi-chip module (MCM) to form an MCMpackage, comprising: providing said PCB system board for said multi-chipmodule (MCM) with a mechanical stiffener for removing excess heat fromsaid MCM to said mechanical stiffener, and applying a thermo-conductinggel package formed a self-conforming insulator structure shaped like acushion of two separate insulating layers filled with a non-compressiblethermo-conducting gel paste insulating low temperature curing epoxyresin material between said PCB system board and said mechanicalstiffener for forming an epoxy interface between the PCB system boardand the mechanical stiffener that also acts as a thermo path for excessheat from the MCM to the mechanical stiffener and causing said gelpackage to conform to all irregularities and to apply equal contactpressure to a printed circuit board surface after assembly of saidthermo-conducting gel package between a mechanical stiffener and saidprinted circuit PCB system board by applying pressure on saidnon-compressible gel package to cause said non-compressiblethermo-conducting gel package to conform and fill any voids between saidprinted circuit and the mechanical stiffener, wherein pressure on saidnon-compressible thermo-conducting gel package is applied by mounting anMCM module to said printed circuit board against said mechanicalstiffener when said gel package is placed between said printed circuitboard and said mechanical stiffener and said MCM module and mechanicalstiffener are drawn together pressing to cause said thermo-conductinggel package to press against said printed circuit board and fill anyvoids between said printed circuit board and the mechanical stiffenerand to cause said gel package to maintain compression between saidprinted circuit board and mechanical stiffener in said MCM package. 2.The method according to claim 1 wherein said low temperature curingepoxy resin is applied as a gel package comprising non-compressibleepoxy.
 3. The method according to claim 2 wherein the gel package isprovided with an activator agent present within said gel package thatcauses the epoxy to cure once the activator is effectively activated. 4.The method according to claim 2 wherein the gel package is provided withan epoxy curing agent that is pressure sensitive and activated when thepackage is compressed between the PCB system board and mechanicalstiffener during assembly.
 5. The method according to claim 2 whereinthe gel package is provided with an epoxy curing agent self-contained inan enclosed package within the gel package.
 6. The method according toclaim 2 wherein the gel package is provided with an epoxy curing agentreleased within the gel package by an adjustment made to release theepoxy curing agent and to cause the epoxy to cure within the gelpackage.
 7. The method according to claim 2 wherein said low temperatureepoxy is cured by heat generated by circuits operating within said MCM.8. The method according to claim 1 wherein said low temperature curingepoxy resin forms a rigid epoxy interface between said PCB system boardand said mechanical stiffener.
 9. The method according to claim 5wherein curing agent is released into said epoxy when said enclosedpackage is ruptured by pressures created in assembly.
 10. The methodaccording to claim 5 wherein curing agent is released into said epoxywhen said enclosed package is ruptured by physical manipulation duringassembly with a sharp point activator.
 11. The method according to claim5 wherein said enclosed package for said curing agent releases saidcuring agent in into said epoxy intermixing therewith when secondarypackage is ruptured by physical manipulation of a decal to cause releaseof said curing agent during assembly.